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Desktop Pick-&-Place Machine: An EETimes Community Project

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Aeroengineer
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Re: Positioning / verification
Aeroengineer   6/21/2014 3:09:37 PM
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It is a whole lot easier to reply to comments when I am not working 12-14 hours a day. 

 

I wanted to elaborate a bit on some of the things that you have brought up in this comment.  The alignment of components will be critical to be able to push into the smaller component sizes and pad spacing. 

 

I see this as a two pronged effort.  The first effort is to ensure that the electromechanical design can actually provide the accuracy and precission that is required for the task.  I have seen a lot of systems that are not designed for this, and they suffer.  Even in the 3D printer world, many of these groups only speak of the min step size, but not about the repetability of their process.  I think that this causes a lot of confusion and is perhaps borderline false advertising. 

 

The other prong of this effort is to look at how to integrate a camera vision system to help in the alignment of the component. 

Aeroengineer
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Re: Desktop PnP--some inspiration
Aeroengineer   6/21/2014 1:54:37 PM
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I have a bit more time now that I can elaborate on my previous comment.  One of the reasons that I think that they were having some issues for paste dispensing is that they were using a pneumatic method.  While this is common, there are issues of compressability and the fact that you need to control the built up pressure in the system in a way similar to how you would control an inertial device.  Added to this, there is a lot of flex in the tubing and such that connects the pump to the syringe.  This is greatly influenced by temperature.  This means that efforts to calibrate it are going to be difficult.  I can also imagine, but I have not actually looked at in depth, that there are differences in viscosity in the solder pastes and that these too can change with respect to temperature. 

 

To combate these, I am thinking of one of two things.  The first, if going with the airpump method, there is a need to reduce the amount of line between it and the syringe.  I would look at doing that by mounting the pump right to the head of the divice.  There are a lot of small pumps that would be suitable for this.  The other option is to go to a mechanical system.  The mechanical system is going to be the most precise as it does not suffer from the "inertial" effects of the pressure system, nor the variability.

Aeroengineer
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Build Area Size?
Aeroengineer   6/21/2014 1:47:59 PM
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I am wondering, I have not heard too many comments about the build area size that people would be interested in for this type of machine.  I would love to hear your thoughts and why you think that it should be that size.

Aeroengineer
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Re: Desktop PnP--some inspiration
Aeroengineer   6/20/2014 8:14:09 PM
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A lot of what you are talking about gets to the heart of the mechanical design which governs accuracy and precission.  Calibration also plays a part.  If the machine has a constant offset, this will continue to propagate into the component placement.  I feel that many of these things can be overcome with good electromechanical design.

Etmax
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Re: Desktop PnP--some inspiration
Etmax   6/20/2014 1:46:46 PM
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I was talking to a company that made a small desktop machine and they said that doing 0603's was about the limit for paste dispensed from a pneumatic driven syringe, and that finer pitch IC's than 0.8mm were also a problem. I for one would be happy to place all the fine pitch IC's and QFN's by hand and then us the machine to place all of the jellybeans. Currently I do the lot with tweezers for prototypes. DFN's are a breeze to do by hand as are TQFP's, the only things I have limited success with is BGA's. If I used a paste mask they would probably work out, the solder on the balls seems to be unleaded which has such lousy flow characteristics.

Thinking_J
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Re: Ariel Electronics Circuit Writer
Thinking_J   6/19/2014 5:28:52 PM
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Many manufacturers of membrane switches use conductive polymers for their circuit traces.. and Ariel's solution would be great for their prototypes. But most of the rest of the industry can't tolerate the higher resistivity of this material for traces.

Low current, low speed, low density.. generally will work fine using conductive polymers.

But even my simpliest designs today often involve USB 2.0 lines (with controlled impedances) or modest levels of current (0.5 amp). All point back to copper.

 

 

betajet
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Ariel Electronics Circuit Writer
betajet   6/19/2014 4:50:01 PM
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Regarding conductive epoxy, here's a repeat of a comment I made last year:

Around 1990 there was a promising attempt to use CNC technology to make circuit boards by Ariel Electronics (California).  They created a gadget called the Circuit Writer which extruded conductive plastic wires onto a substrate, basically a 2-D plotter with an extrusion head.  I actually visited Ariel and saw a Circuit Writer working.  I don't think the technology got anywhere, but maybe it was just ahead of its time and with newer 3D extrusions this could be done practically.  For more info, Google "ariel electronics circuit writer".

Thinking_J
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Re: Interesting
Thinking_J   6/19/2014 4:48:30 PM
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@Salbayeng.

Your comments show much experience. Agreed - conductive polymers are not going to replace solder for every application.

Like so much of life... the devil is in the details.

Cost: I can get both solder and conductive polymers cheaper than what you quoted... and what ever it is today, will be different tomorrow. Often the difference is based on where you are.

Shelf life: similar observations to above (we don't like to keep solder for more than 6 months and never reuse by placing excess back into storage)

Curing: agree.. some curing heat is generally required. But much lower temps than solder. Especially the higher temps of ROHS solders.

Smearing: correct can be easier to be corrected when using solder. But always a bad situation. Even solder balls create problems.

Wicking (surface tension): this depends on the pcb surface finish. OSP (organic surface protectant) type finish,  solder doesn't generally wet beyond where you put it. ENIG or HASL variations generally the solder will wet entire surface.
And, yes , the surface tension can re-align a component with solder. Some designs and facilities depend on this characteristic of solder. But if often this creates as many problems as it solves (tomb stoning, floating off center because on thermal issues, solder balls floating around in the assembly).  Better solution: just put the part where it belongs... and expect it to stay there. And don't bump it until it is cured or reflowed! I have seen articles by "experts" claiming the lack of wetting and automatic re-alignment with many new ROHS solders helps them reduce solder bridging! .. go figure...everyone seems to view this differently.

An item many have ignored: ROHS vs NON_ROHS compliance on components, specifically the surface finish of the connecting points. It is generally not recommended to mix soldering materials and component types. This is not an issue with conductive polymers.

 

Aeroengineer
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Re: Interesting (limiting uses of conductive polymers)
Aeroengineer   6/19/2014 4:12:03 PM
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Once again, thank you for the detailed information.  This provides interesting food for thought not just for this effort, but for efforts to manufacturer other PCBs that are in production.

Thinking_J
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Re: Interesting (limiting uses of conductive polymers)
Thinking_J   6/19/2014 3:52:11 PM
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Zeeglen: I understand your basic concern: thermal / electrical performance of conductive polymers comparted to solder (primary metal: tin .. assuming ROHS compliance).

You are correct .. conductive polymers are not appropriate for everything. But I was directing my thoughts to whom I thought would be using a table top machine with all the processing done at one station... for prototying or very small production runs.

Generally,

- Thermal conductivity:

Yep.. metal is better. But most applications don't approach the limits of either material.

- Resistance:

The differenences over 0.0005-0.001" thickness in the junction typically larger than 20 sq mil, are minor. Assuming use of quality conductive epoxy intended for this application. Unless you are sensitive to variations of less than 0.00001 ohms (you might be).. I don't think it will matter much. Many are not aware of the variations they may be experiencing with solder on current sense resistors. Good pcb layout methods are the primary concern for current sensing.

- Mechanical differences:

Many are not aware of the sensitivity of the some of the components to variations in connection process to the mechanical strength of the joints. Ceramic capacitors with high plate density (capactitance per volume) often demonstrate large deviations in quality due to this.

Example: the measured shear strength of a soldered X7R chip capacitor varies significantly with capacitance for a given size.
 I have measured a range of 2-17lbs for single location/size. While the same location/size with conductive epoxy showed very little standard deviation from 7 lbs of shear strength.

It was determined the capacitor's end platng (based on density) was the primary reason for the large std dev in the soldered joint performance. And the primary reason for this sensitivity to the quality of the capacitor end plating was exposure to higher temperatures and sensitivity to flux activation level (all mild).

Because of the physical space to work with and the minimum capacitance over temperature extremes and voltage required.... we had to work with capacitors that were (at the time) near the limits of how much capacitance we could get in a given volume (size).

I couldn't find a capacitor manufacturer that didn't display some form of this problem. I couldn't tolerate less than 5lbs shear strength on ANY product (avionics -55.+125C rating with high shock and vibration requirements). So, the ONLY solution at the time was to replace solder process with high temp rated conductive epoxy.

Connection junctions are typically a very small fraction of a square. (bulk resistance of a given material, generally specified in "squares" with a given thickness or resistance per cm)

Which is a very, very different animal compared to conductive epoxies vs copper used for traces (many squares) , often an issue when using bendable circuitry (copper vs conductive polymers).

 

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